In the trucking industry, trailers may be coupled together into tandem or triple unit combinations and hauled over the road by tractor units.
When multiple trailers are coupled together into tandem or triple combinations, the safety lighting on the rear of the intermediate trailers is blocked from view by the last trailer. However, these lights continue to operate and draw current from the trailers' electrical supply.
In most instances, all electrical systems on the tractor-trailer combination are powered by the tractor's engine-mounted alternator, with starting and stand-by power from a storage battery. Electrical power from the tractor is distributed to the trailer combination through a standardized multi-circuit cable to provide interchangeability between tractors and trailers. Per trucking industry convention, seven circuits are color coded and oriented radially within the cable and receptacle plugs. With this established convention, control of power transmission into any one of the seven circuits can be maintained at the plug/receptacle interface.
To continue the electrical power cable interconnecting the lead trailer to any following trailers, a matching receptacle is positioned at the rear of all trailers intended for tandem or triple combinations. This rear receptacle is connected to the front mounted trailer receptacle of the next trailer in line by a color-coded wiring harness. As a result, two sets of trailer lighting in a tandem combination, or three sets of trailer lighting in a triple combination, draw current from the tractor.
In addition to safety lighting, there is a growing use of anti-lock braking systems (ABS) on trailers, as regulations requiring trailer ABS are mandatory. The ABS utilize electrical power to sense, apply and control the trailer air-brake system, which is activated by the tractor driver. Regulations governing ABS on tractor trailers require a new inter-related brake system that requires both electrical and air power supplied by the tractor. In the ABS equipped combinations, not only the trailer, but also the connecting converter dollies will require electrical power to activate the air brake systems and trailer safety lighting. The primary ABS electrical power is to be provided by a separate full time continuous power circuit. A second, back-up ABS power source is to be supplied by the industry standard circuit which currently only powers the trailer and dolly stop lights.